The present invention is generally directed to a spring loaded valve for use with cryostat inserts for adding cryogenic liquid to cryostats.
In the generation of images in medical magnetic resonance diagnostic systems, it is necessary to provide a temporarily stable and spatially homogeneous magnetic field. The use of superconductive electrical materials maintained at a temperature below their critical transition temperatures, provides an advantageous means to produce such a field. Accordingly, for such MR imaging devices, a cryostat is employed. A cryostat contains an innermost chamber in which liquid helium, for example, is used to cool the superconductive materials. The cryostat itself, typically comprises a generally toroidal structure with other nested toroidal structures inside the exterior vessel, to provide the desired vacuum conditions and thermal shielding. Since it is necessary to provide electrical energy to the main magnet coil and to various correction coils employed in MR imaging and to replenish coolant material, it is necessary that there be at least one penetration through the cryostat vessel walls.
Present MR magnets with horizontal tube penetrations require two separate plugs, one to power the magnet as described and claimed in U.S. Pat. No. 4,535,596, and another plug to be used when the magnet is operating in the persistent mode providing high thermal efficiency as described and claimed in U.S. Pat. No. 4,516,404. The procedure to remove and insert these plugs is complex, requires the training of operators and can inadvertently admit air inside the cryostat. The use of a helium liquifier which captures helium boiloff and returns liquid helium to the cryostat, with these plugs could result in contamination of the liquifier because of the admission of air into the system.
During initial magnet cooldown, it is sometimes desirable to fill the helium vessel by directing liquid helium to the bottom of the vessel, rather than having it pour in from the top to reduce helium consumption. When the two separate plugs are used a pipe having a blocked end and a side hole is inserted in the cryostat penetration. The blocked end portion prevents the liquid for pouring into the top of the vessel and the side hole is in flow communication with a pipe leading to the bottom of the vessel. After initial cooldown the plug of U.S. Pat. No. 4,535,596 has a pipe which directs coolant to the top of the helium vessel when additional coolant is added. The pipe has an opening at its end and covers the pipe leading to the bottom of the vessel.
It is an object of the present invention to provide a spring loaded valve for use with a cryostat insert for selectively directing liquid cryogen to the bottom or top of a supercoolable vessel.